Conventional membrane switches are typically manufactured individually by laminating several independent elements with interposed double-sided adhesive sheets. The steps of die cutting, lamination, and assembly are repeated multiple times during manufacturing leading to a labor intensive and slow process. The typical elements of a membrane switch include a graphic layer, laminating adhesive, embossed electrical contactors, spacer, electrical contact, laminate adhesive, and backing. These elements are individually manufactured, individually die cut and assembled layer by layer. Additionally, in many cases additional steps are required when adding an electroluminescent lamp and/or LED to backlight the switches. Additional steps are required to provide tactile feel using metal domes, poly domes, or magnetic switches. Indicator lights, and digital or alphanumerical displays are also often used either as a part of the membrane switch or adjacent to the switch.
Referring to FIG. 1, an exploded view of a conventional membrane switch using electroluminescent lamp technology is illustrated, and is generally identified by the numeral 20. Layer 22 is a substrate with a printed graphic element 24. A typical substrate layer 22 is made of polyester or polycarbonate with thicknesses of 3 to 7 mils. The graphic element 24 is usually on the bottom face so that substrate 22 will protect the graphic element 24. Typically, graphic printing is completed in a batch process. The printing flow is broken up by the operation of die cutting. This cut out piece that typically includes substrate layer 22 and graphic element 24 is called a graphical overlay.
Layer 26 is an electroluminescent lamp printed on an Indium Tin Oxide (ITO) sputtered substrate. The substrate is typically polyester or polycarbonate, 3 to 5 mils thick. The substrate is sputtered with ITO. The ITO sputtered substrate is screen printed with the following layers: Silver ink bus bars 0.5 to 1.0 mils thick, Phosphor 1 to 1.5 mils thick, Dielectric layer containing barium titanate 0.2 to 0.6 mils thick, back electrode of silver or graphite filled inks 0.5 to 1 mils thick, insulating layer 2 to 6 mils thick. Once the lamp layer 26 has been successfully printed, it is die cut from the substrate.
Layer 22 and the lamp layer 26 are joined together in a laminating step. Layer 28 is a double-sided laminating adhesive and is die cut to the same size as the layer 22 and lamp layer 26. The double-sided laminating adhesive layer 28 attaches the lamp layer 26 to the layer 22. Alignment and removal of air bubbles are critical in lamination steps and are serious sources of defects.
A conductive contact element layer 30 is used to actuate the switches. This layer may include metal domes, polymer domes coated with a conductive layer or flat electrical contactors. The electrical contactors are used when a simple electrical contact is needed. The purpose of metal domes and poly domes is to give a tactile response when the switch is depressed. Conductive layer 30 is connected to lamp layer 26 using an adhesive layer 32.
Layer 34, the electrical circuit and contact points for the switch, is composed of a substrate of polyester or polycarbonate 3 to 7 mils thick. A first layer of conductive ink is printed on the substrate. These inks are often made with silver or graphite as the conductive elements. If more than one conductive layer is needed, an insulating layer is printed next to protect the first conductive layer. A second conductive layer is then printed. After successfully completing these steps the circuit layer 34 is then die cut.
A spacer layer 36 is also die cut. The spacer layer 36 is approximately the same thickness as the metal domes and has adhesive on both sides. After die cutting the spacer layer 36, layer 36 and the circuit layer 34 are laminated together. Metal domes 38 are then placed in the holes 40 of the spacer layer 36 either manually or by a pick and place machine. Conductive layer 30 is applied over the spacer layer 36 and laminated into place.
The metal domes 38 and electrical circuit layer 34 are laminated to the conductive layer 30 using a double-sided laminating adhesive layer 36. Adhesive layer 36 is die cut to the proper size before the lamination step.
A final laminating adhesive layer 42 is applied to circuit layer 34. The laminating adhesive layer 42 is die cut into the desired shape and is applied to the back of the electrical circuit layer 34. A release liner layer 44 is left on the laminating adhesive until the finished membrane switch 20 is applied to its final location on a circuit board or electronics enclosure.
In addition to the labor necessary to assemble these many different layers (FIG. 1) there are significant quality and manufacturing issues that arise from the lamination steps required to produce a conventional membrane switch. These include, but are not limited to, die cut registration, alignment of the various layers, and removal of air trapped in the lamination process. Because the membrane switches are die cut each individual membrane switch must be processed one at a time.
Moreover, the placement of discreet lighting elements such as light emitting diodes, the connection of these elements to electrical traces with the use of conductive polymers, and the curing of these polymers are all very labor intensive operations. These operations steps may not be part of the membrane switch manufacturer's process. Hence, the manufacturer may outsource these operations to a third party vendor resulting in a disruption of the normal manufacturing flow.
When electroluminescent lamp lighting is used it is advantageous to place both the graphic and the lamp behind the deformable substrate. The deformable substrate is typically composed of either polyester or polycarbonate material that is very rugged and durable to environmental conditions. Common sources of electroluminescent lamp lighting do not allow graphics to be printed directly between the substrate and the optically transmissive conductive layer of the lamp nor do they permit graphic layers to be printed between the ITO and other layers of the lamp. This is because the graphic layers interfere with the electrical connection to the ITO conductive layer often used on the substrate and/or the graphic layer may contaminate other clear conductive layers that may be used instead of ITO.
Therefore, a need exists for combining electroluminescent lamp technology and membrane switch elements into a continuous manufacturing process that eliminates the conventional batch process used for lamination steps and the labor required to assemble the layers of the switch while protecting the graphics.